The biosynthesis of ACTH, endorphin, alpha-MSH, vasopressin and oxytocin, with emphasis on the post-translational processing enzymes which cleave their respective prohormones, have been studied. Converting enzymes which specifically cleave at paired basic residues of pro-opiocortin and pro-vasopressin/pro-oxytocin have been detected in the secretory granules of the anterior and intermediate lobe and the posterior (neural) lobe of the pituitary, respectively. These enzyme activities have been partially characterized and purified. A carboxypeptidase-B-like enzyme activity which functions to remove the basic residues from the C-terminal of the peptide hormone following the action of the prohormone converting enzyme, has been detected in anterior, intermediate and posterior lobe secretory granules. An acetylation enzyme which acetylates both desacetyl Alpha-MSH and Beta-endorphin has been detected in the intermediate lobe secretory granules. The regulation of acetylation and release of Alpha-MSH was studied in the toad intermediate lobe.